The present invention relates to methods of driving electric discharge display panels used as image displays of personal computers, office work stations, or hanged television sets with future development expectation, etc. and, more particularly, to methods of driving electric discharge display panels having double side discharge electrodes, which permit ready manufacture of panels having high capacity and very fine structures.
Electric discharge panels usually are simple in construction and readily permit panel face area increase, and they further permit use of inexpensive soda glass extensively applied to window glasses and the like as their substrate.
An electric discharge display panel is formed by using two transparent insulating substrates of soda glass, forming partitioning walls or the like on these substrates for defining electrodes and pixels as units of display on the substrates and bonding together the two substrates with the partitioning walls. Gas for electric discharge is sealed in the space defined in the bonded structure. The partitioning walls usually have a height of about 0.2 mm, and the transparent insulating substrates have a thickness of about 3 mm. It is thus possible to obtain very thin and light-weight displays.
Such electric discharge display panels are roughly classified to DC type and AC type in dependence on their panel structure. In the DC type, the electrodes are in direct contact with gas, and once discharge is caused, DC current flows continuously. In the AC type, on the other hand, an insulating layer intervenes between the electrodes and discharge gas, and current is caused in a pulse-like form for a short period of about one microsecond after voltage application before it is converged. In this case, the current caused is restricted by the electrostatic capacitance of the insulating layer. The insulating layer serves as a capacitor, and by applying AC pulses recurrent light pulses are emitted for display.
Although the DC type is simple in structure, the electrodes which are directly exposed to the discharge are soon worn out, so that it is difficult to obtain long life of the electrodes. Although the AC type requires considerable man-hour and expenditure for the insulating layer formation, long life of electrodes can be obtained because the electrodes are covered by the insulating layer. Besides, this type readily permits realizing a function called memory, which permits high intensity light emission.
The structure of an AC memory type electric discharge display panel, and also a method of and a prior art circuit for driving the structure, will now be described. FIGS. 12(a) and 12(b) show an AC memory type electric discharge panel having a surface discharge type electrode structure, as disclosed in Japanese Laid-Open Patent Publication No. 7-295506, FIG. 12(a) being a plan view, FIG. 12(B) being a sectional view taken along line X-X'.
The electric discharge display panel shown in FIGS. 12(a) and 12(b) carries an electric discharge panel structure, constitutes part of a discharge gas vessel, and permits display light to be taken out from it. To these ends, the display panel comprises a first transparent insulating substrate 11 of soda glass about 3 mm in thickness, and a second insulating substrate 12 of the same soda glass about 3 mm in thickness in parallel to and spaced apart a predetermined distance from the first insulating substrate 11.
On the first insulating substrate 11 are formed pluralities of alternate transparent NESA film scan and sustained discharge electrodes 13a and 13b parallel to the fist insulating substrate 11, metal electrodes 13c constituted by a thick silver film formed on the scan and sustained discharge electrodes 13a and 13b for supplying sufficient current thereto, an insulating layer 18a constituted by a thick transparent glaze film covering the scan, sustained discharge and metal electrodes 13a to 13c, and a protective film 19 of MgO, 2 .mu.m in thickness for protecting the insulating layer 18a from discharge. Since the scan and sustained discharge electrodes 13a and 13b are formed on the same surface, they are collectively referred to as double discharge electrodes.
On the second insulating substrate 12 are formed a plurality of column electrodes 14 constituted by a thick silver film, an insulating film 18b constituted by a thick film covering the column electrodes 14 and the second insulating film 12, a partitioning wall 16b constituted by a thick film for ensuring a discharge gas space and partitioning pixels, and phosphor 17 constituted by Zn.sub.2 SiO.sub.4 :Mn for converting ultraviolet radiation generated by electric discharge in discharge gas to visible light.
The two insulating substrates 11 and 12 with the above structures formed thereon are bonded together, thereby forming a discharge gas space 15 defined between them. The discharge gas space 15 is filled with discharge gas, such as a mixture of He and Ne in a ratio of 7 to 3 with a 3% Xe content, under a total pressure of 500 Torr.
As shown in FIG. 12(a), sections enclosed by vertical and horizontal lines of the partitioning wall 16, constitute pixels 20 forming discharge cells. To obtain an electric discharge display panel capable of full color displaying, the phosphor 17 shown in FIG. 12(b) is coated in three colors, i.e., red, green and blue, for the individual pixels. The display direction of this electric discharge display panel may be either upward or downward in FIG. 12(b). In this case, however, the downward display direction is preferred or this direction provides a style that the light-emitting part of the phosphor is viewed directly and emits higher brightness to be obtained.
FIG. 13 is a plan view showing of the electrodes of the electric discharge display panel. Referring to the Figure, the pixels 20 are provided at intersections of the scan electrodes S.sub.i (i=1, 2, . . . , m) and the column electrodes D.sub.i (i=1, 2, . . . , n). Designated at 10 is the electric discharge display panel, 21 a seal section, along which the first and second insulating substrates 11 and 12 are bonded together to define a sealed space, which is filled with discharge gas, C.sub.1, C.sub.2, . . . , C.sub.m sustained discharge electrodes 13a, S.sub.1, S.sub.2, . . . , S.sub.m scan electrodes 12b, and D.sub.1, D.sub.2, . . . , D.sub.n-1, D.sub.n column electrodes 14.
An actual electric discharge display panel, in the case of VGA system, for instance, has 480 scan electrodes S.sub.1, S.sub.2, . . . , S.sub.m, 480 sustained discharge electrodes C.sub.1, C.sub.2, . . . , C.sub.m, 1,920 column electrodes D.sub.1, D.sub.2, . . . , D.sub.n-1, D.sub.n. The pixel pitch is 0.35 mm as column electrode pitch and 1.05 mm as scan electrode pitch. The scan electrodes are spaced apart from the column electrodes by a distance of 0.2 mm.
Now, a method of gradation display using the above electric discharge display panel will be described. With an electric discharge display panel, unlike other devices, it is difficult to obtain high brightness gradation display by updating applied voltage. Usually, the gradation display is obtained by controlling the number of light emission times. Particularly, a sub-field method as will be described later is used for high brightness gradation display.
FIG. 14 is a view for explaining a drive sequence in the sub-field method. In the Figure, the ordinate is taken for scan electrodes, and the abscissa is taken for time. As is shown, one frame of image is transmitted in one field. The period of one frame varies with computers and broadcast system, but in many cases it is set roughly in a range of 1/50 to 1/75 sec.
In the case as shown in FIG. 14, in the gradation image display on an electric discharge display panel one field is divided into k sub-fields SF1 to SF6. Each sub-field comprises a write time, in which display data with preliminary discharge pulses, preliminary discharge erasing pulses, scan pulses, data pulses, etc., and a sustained discharge period for display light emission. It is possible to omit the preliminary discharge pulses and preliminary discharge erasing pulses in the write period.
The light emission intensity of each pixel is controlled by weighting the number of light emission times of sustained discharge in each pixel in each sub-field with a weight factor of 2.sup.n, as expressed by a formula. ##EQU1## where n is the rank number of sub-field such that it represents the lowest intensity sub-field when it is "1" and the highest intensity sub-field when it is "k", L.sub.1 is the intensity of the lowest intensity sub-field, and an is a variable taking either value "1" or "0" such that it is "1" in case when causing light emission of the pertinent pixel in n-th sub-field and "0" in case when causing no light emission. Since the light emission intensity varies with the sub-fields, the intensity control can be obtained by selecting either light emission or no light emission in each sub-field.
In the case of FIG. 14 in which k=6, when obtaining color display with a red, a green and a blue pixel as a set, a display in 2.sup.k =2.sup.6 =64 gradations can be obtained in each color. A number of colors (including black) to be displayed is 64.sup.3 =262144. In the case of k=1, in which one field is equal to one sub-field, a display in two gradations (i.e., either "on" or "off") can be obtained in each color. A number of colors (including black) to be displayed is 2.sup.3 =8.
FIG. 15 is a graph showing an example of drive voltage waveforms and light emission waveform in one sub-field in the case of the electric discharge display panel shown in FIGS. 12 and 13.
In the Figure, labeled (A) is the waveform of voltage applied to the sustained discharge electrodes C.sub.1, C.sub.2, . . . , C.sub.m, (B) the waveform of voltage applied to the scan electrode S.sub.1, (C) the waveform of voltage applied to the scan electrode S.sub.2, (D) the waveform of voltage applied to the scan electrode S.sub.m, (E) the waveform of voltage applied to the column electrode D.sub.1, (F) the waveform of voltage applied to the column electrode D.sub.2, and (G) the waveform of light emission of the pixel a11. The pulses shown with oblique line in the waveforms (E) and (F), are either provided or not in dependence on whether or not to write any data. The data voltage waveforms shown in FIG. 15 are such that data are written in pixels a.sub.11 and a.sub.22, and that display in the third and following columns of pixels is made in dependence on whether data is present or not.
To the sustained discharge electrodes C.sub.1, C.sub.2, . . . , C.sub.m are applied sustained discharge pulses 31 and preliminary discharge pulse 36. To the scan electrodes S.sub.1, S.sub.2, . . . , S.sub.m, scan pulses 33 are applied line sequentially at timings independent on the individual scan electrodes, in addition to the common pulses, i.e., sustained discharge pulses 32, erasing pulses 35 and preliminary discharge erasing pulses 37. To the column electrodes D.sub.i (i=1, 2, . . . , n), data pulses 34 are applied in synchronism to the scan pulses 33 in the case of presence of light emission data.
In the electric discharge display panel shown in FIGS. 12 and 13, the discharge of the pixels that have emitted light in the immediately preceding sub-field is first erased with the erasing pulses 35. Then, all the pixels are forcibly preliminarily discharged at a time with the preliminary discharge pulse 36. The preliminary discharge is then erased with the preliminary discharge erasing pulses 37. In the above, write discharge with scan pulses to be applied next is facilitated.
After erasing the preliminary discharge, by causing write discharge by applying the scan pulses 33 and data pulses 34 at the same timing between the scan electrodes and the column electrodes, discharge is caused between the scan electrodes and the column electrodes simultaneously with the write discharge. This discharge is called write sustained discharge. Subsequently, sustained discharge is sustained between adjacent scan and sustained discharge electrodes by the sustained discharge pulses 31 and 32. When the sole scan pulses 33 or the sole data pulses 34 are applied, neither write discharge nor subsequent sustained discharge is caused. This function is called memory function, and the light emission intensity of each sub-field is controlled according to the number of times the sustained discharge is caused.
In the prior art structure described above, a pair of sustained discharge electrode 13a and a scan electrode 13b pass through each pixel. However, from the standpoint of realizing finer structures, the number of electrodes involved is suitably as small as possible. This is so because the smaller the number of electrodes the more the panel failure due to electrode breaking can be reduced. The reduction of the numbers of the sustained discharge and scan electrodes 13a and 13b is also desired because the metal electrodes 13b behave obstructively against the operation of taking out emitted light.
To solve the above problems, an electric discharge display panel and a driving method of the same are disclosed in Japanese Laid-Open Patent Publication No. 2-220330. FIGS. 16(a) and 16(b) show the electric discharge display panel disclosed in the publication, FIG. 16(a) being a plan view, FIG. 16(b) being a fragmentary sectional view.
As shown in FIGS. 16(a) and 16(b), the discharge panel comprises a first insulating substrate 51 of an insulating material, a plurality of discharge electrodes 52 and 55 formed on the first insulating substrate 51 such that they are parallel thereto, a dielectric layer 57 covering the discharge electrodes 52 to 55, a partitioning wall 56 formed on the discharge electrodes 52 to 56 such as to longitudinally divide each thereof into two parts, a partitioning wall 63 formed on top of the partitioning wall 56, an insulating layer 62 formed on the partitioning wall 63, address electrodes 61 formed on the insulating layer 62 such as to cross the discharge electrodes 52 to 55, and a second insulating substrate 60 facing the first insulating film 51 and defining a gas discharge space together therewith. Spaces defined by the partitioning walls 56 and 63 constitute unit cells (pixels) 59.
The discharge electrodes 52 to 55 consists of three different kinds of electrodes, i.e., Y discharge electrodes 53 and 55 occurring as every other electrode, and X.sub.1 and X.sub.2 discharge electrodes 52 and 54 occurring alternately between adjacent Y discharge electrodes 53 and 55. The frequency of the sustained discharge pulses applied to the Y discharge electrodes is set to double that applied to the X.sub.1 and X.sub.2 discharge electrodes, such that the pulses for the X.sub.1 and X.sub.2 discharge electrodes are alternately coincident in phase with the pulses for the Y discharge electrodes. Thus, AC sustained discharge voltages of opposite polarities are applied alternately to two adjacent display lines between a common Y discharge electrode and respective X.sub.1 and X.sub.2 discharge electrodes.
In this electric discharge display panel, the sustained discharge is caused between adjacent electrodes as shown by arrow a or a'. This means that only a single surface discharge electrode (i.e., X.sub.1, X.sub.2 or Y discharge electrode) is formed for each pixel column on the first insulating substrate 51. In other words, the electrode density may be one half compared to the prior art example shown in FIG. 12. The scan and sustained discharge electrodes which are each used for two pixels on both sides are called double side discharge electrodes.
In the above prior art electric discharge display panel and the method of driving the same, however, the Y discharge electrodes for applying scan pulses to them are each interposed between two adjacent pixel columns. Therefore, extremely complicated drive waveforms are necessary for sequentially scanning the Y discharge electrodes to write display data and also causing sustained discharge.